As used in telecommunications, a frequency multiplexer (hereafter referred to simply as “multiplexer”) is a network that separates signals from or to a common port to or from other ports, sorted according to their frequency. Frequency sorting is conventionally achieved by a filter in each signal line between the common port and another port, which filter allows the signal for the associated signal line to pass while attenuating the other signals. The filters generally may be low-pass, band-pass, or high-pass filters as appropriate.
Multiplexers may be used in a mixer for up-converting or down-converting signals in a radio-frequency transmitter or receiver. In such an application, the multiplexer may be a triplexer. Generally, a mixer performs frequency conversion by multiplying two signals. For example, in a receiver, a radio-frequency signal and a local-oscillator signal may be multiplied to produce an intermediate-frequency or baseband signal. (Baseband signals, for which there may be no lower frequency limit, will be considered intermediate-frequency signals for the purpose of this discussion.) Similarly, in a transmitter, an intermediate-frequency signal and a local-oscillator signal may be multiplied to produce a radio-frequency signal.
Isolation between the various signal lines is important. Conventionally, a low-pass filter may be used in the intermediate-frequency signal line to allow substantially only the intermediate-frequency signal to be conducted. A band-pass filter may be used in the local-oscillator signal line to attenuate intermediate- and radio-frequency signals. A band-pass or high pass filter may be used in the radio-frequency signal line to attenuate intermediate- and local-oscillator signals. Additionally, the signal path to a common terminal desirably has low impedance for the signals in each of the signal lines.